Piezoelectric acoustic devices are of great interest for radio frequency (RF) signal processing. Because these devices efficiently convert electromagnetic waves to acoustic waves (and back), the geometrical area that is required for signal processing is reduced by a large factor, comparable to (c/v)2, where c is the speed of light and v is the speed of sound. This factor is very large, typically on the order of 109. This has led to very substantial miniaturization of passive components such as resonators, filters, and delay lines.
It has long been known that the acoustic waves in piezoelectric acoustic devices are intrinsically accompanied by evanescent electric fields. Since the late 1960s, it has also been known that these evanescent fields can interact with traveling semiconductor currents in a manner that produces gain. This gain depends sensitively on the propagation velocity of the charge carriers relative to the acoustic wave. Indeed, if the acoustic wave and the charge carriers propagate in opposite directions, the system experiences attenuation rather than gain. Hence, the gain is highly non-reciprocal.
Early investigators of hybrid semiconductor-piezoacoustic systems, notably in the 1960s to the 1980s, demonstrated highly nonreciprocal amplifiers, oscillators, and even correlators. Some of these devices had great promise; demonstrations included amplifiers with tens of decibels of gain per millimeter and delay lines with 100 dB of non-reciprocity.
However, these systems were not competitive in noise performance with transistor-based active RF components, and interest consequently waned. The source of excess noise was well understood, but it was also understood that a practical level of performance could not be achieved with the available materials and fabrication capabilities of the day.
However, recent advances in Lamb wave piezoacoustic devices, heterogeneous integration of epitaxial semiconductors, and multiphysics simulation have rekindled the hope for new devices that can provide the functionality of the archetypal devices of the 1960s-1980s, but without excess noise.